Effect of CNTs-assisted ball milling on morphology and oxidation behavior of Zr powders

Abstract:

Morphology evolution and oxidation behavior of Zr powders milled with and without CNTs for different time were compared. XRD and SEM were used to determine the phase composition and morphology of obtained Zr and Zr/CNTs powders. Thermogravity (TG) analysis was applied to evaluate the thermal oxidation behavior of Zr and Zr/CNTs. Results showed that the phase composition of Zr after milling was not influenced by adding CNTs, but the morphology and oxidation behavior of Zr were influenced. In case of milling without CNTs, particle size of Zr continued to reduce from more than 10 μm to about 2~3 μm when milling time increased from 1 h to 3 h. In case of CNTs-assisted milling, the particle size reduction of Zr was not obvious until milling time prolonged to 3 h. This was because those CNTs adhered on the surface of Zr or CNTs-induced agglomerations among Zr prohibited the mechanical impact and fracturing effects of milling balls on Zr particles. Compared with direct milling of Zr, when the obtained Zr had similar particle size, adding CNTs lowered the onset oxidation temperature and peak exothermic temperature of Zr powders by 11 ℃~37 ℃, depending on milling time. This was ascribed to the high heat conductivity of CNTs which favored heat transfer between Zr particles. At the same time, oxidative weight gain of Zr/CNTs was slightly lower than the corresponding pure Zr when milling time was less than 2 h and then this trend reversed. The initial decrease in weight gain of Zr in Zr/CNTs mixtures under shorter milling time was related to two factors: one was CNTs-induced agglomerations of Zr, which prohibited the contact of Zr with oxygen and thus decreased the degree of oxidation, and the other was the delayed particle size reduction of Zr in Zr/CNTs mixtures. The subsequent reversal in weight gain for pure Zr and Zr/CNTs was because the positive effect of particle size reduction of Zr in Zr/CNTs on its thermal oxidation just began, while the spontaneous oxidation-induced negative effect for well-refined pure Zr has become very obvious.